Preserve timestamps when copying l10n cdb files

The fix for bug 27320 will have better performance if it can skip
updating languages that haven't actually changed since the last run.
LocalisationUpdate already avoids needlessly updating the cdb files, but
we need to tell l10nupdate-1 to preserve timestamps when copying them.

Change-Id: I72a1557f9c18b845c952dd2e2697d92e8eb71d93